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It's usually because it's tied to some sort of OS-level new feature. In the case of IE10, it's the rendering engine for (some kinds of) Metro style apps. If you're providing infrastructure for some new features, it's easy to rely on other new infrastructure, and pretty soon you can only run on the new OS. It probably was a serious effort to back-port it like this, although it's also possible IE10 represents a new model that changes things.

Because that was the way to put Netscape out of business. By tying the browser with the OS they made sure that every new PC would be running IE. In the long term of course this was an awful decision since upgrades come once every couple of years.

Although I'm a .NET developer I sincerely hope that IE dies. IMHO it is the worst mistake MS ever made, by slowing down upgrade cycles that much they managed to alienate every web developer/designer out there by making their life miserable trying to fix pages that didn't work in IE.

Because the IE component is a core component of the operating system that other apps can rely on.